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Initial flame development and propagation were visualized under different stratified conditions to estimate the effects of stratification on the engine performance in a port injection SI engine. Experiments were performed in an optical single cylinder engine modified from a production engine and images were captured through the quartz window mounted in the piston by an intensified CCD camera. Four cylinder heads, which have different intake ports, were assembled on the cylinder block to vary port swirl ratios. Stratification was controlled by the combination of the port swirl ratio and injection timing. Under each condition, flame images were captured at the pre-set crank angles. These were averaged and processed to characterize the flames. The flame stability was estimated by the weighted average of flame area and luminosity.